1. Field of the Invention
The present invention relates to a magnetic sensing element to be used for a hard disk device and magnetic sensor. In particular, the present invention relates to a magnetic sensing element that allows effective control of the magnetization of a free magnetic layer in defining narrow track width, and a process for manufacturing the magnetic sensing element.
2. Description of the Related Art
FIG. 34 shows a cross section of the structure of a conventional magnetic sensing element viewed from aside opposite a recording medium.
In FIG. 34, the reference numeral 1 denotes a substrate on which a multilayer 8 comprising an antiferromagnetic layer 2, a pinned magnetic layer 3, a nonmagnetic material layer 4 and a free magnetic layer 5 is formed. Hard bias layers 6 are formed in both side areas of the multilayer 8, and an electrode layer 7 is formed on each hard bias layer 6.
Magnetization of the pinned magnetic layer 3 is fixed in the Y-direction as shown in the drawing by an exchange coupling magnetic field generated between the antiferromagnetic layer 2 and pinned magnetic layer 3. Magnetization of the free magnetic layer 5 is aligned, on the other hand, in the X-direction by a vertical bias magnetic field from the hard bias layer 6.
As shown in FIG. 34, while the track width Tw is defined by the width of the free magnetic layer 5 in the track width direction (X-direction), the track width tends to decrease more as the demand for recording density increases with time.
However, advanced narrowing of the track width made it impossible to adequately control magnetization of the free magnetic layer 5 in the structure of the magnetic sensing element shown in FIG. 34 by the reasons as described below.
(1) While the width of the free magnetic layer 5 is reduced by narrowing the track width in the structure shown in FIG. 34, the narrower track width makes the area affected by a strong vertical bias magnetic field from the hard bias layer 6 to account for a large proportion in the free magnetic layer 5. Since the area under the influence of the strong bias magnetic field serves as a dead zone where magnetization is hardly varied against an external magnetic field, regenerative sensitivity is lowered due to an increased proportion of the dead zone as the track width is narrowed.
(2) The region between the hard bias layer 6 and free magnetic layer 5 tends to be a magnetically discontinuous region. This tendency is enhanced when a bias underlayer made of Cr is interposed between the hard bias layer 6 and free magnetic layer 5.
The effect of a demagnetization magnetic field at each outer side in the track width direction of the free magnetic layer 5 is increased by this magnetically discontinuous state, causing a phenomenon, known as buckling phenomenon, that disturbs magnetization of the free magnetic layer 5. This buckling phenomenon appears in a wide area of the free magnetic layer 5 as the track width is narrowed, thereby reducing the stability of regenerative waveform is reduced.
(3) A part of the vertical bias magnetic field from the hard bias layer 6 leaks to a shield layers (not shown) formed on and under the magnetic sensing element shown in FIG. 34 in accordance with the gap narrowing. This leak disturbs the magnetization of the shield layer while weakening the bias magnetic field supplied to the free magnetic layer 5, thereby making it impossible to effectively control magnetization of the free magnetic layer 5.
To overcome the problems as described above, the magnetization of the free magnetic layer 5 has been controlled in recent years by employing an exchange bias method using an antiferromagnetic layer on the free magnetic layer.
The magnetic sensing element using the exchange bias method is manufactured, for example, by the manufacturing processes shown in FIGS. 35 and 36. FIGS. 35 and 36 are partial cross sections of the structures of the magnetic sensing elements viewed from a side opposite a recording medium.
In the manufacturing process shown in FIG. 35, an antiferromagnetic layer 2 comprising a Pt—Mn alloy is formed on a substrate 1, followed by laminating a pinned magnetic layer 3 made of a magnetic material, a nonmagnetic material layer 4 and a free magnetic layer 5 made of a magnetic material thereon. A Ta layer 9 is formed on the free magnetic layer 5 in order to protect the surface of the free magnetic layer 5 from being oxidized when it is exposed to the atmosphere.
Then, a lift-off resist layer 10 is formed on the Ta layer 9 shown in FIG. 35, and all the Ta layer 9 exposed at each side in the track width direction without being covered with the resist layer 10 is removed by ion-milling. A part of the free magnetic layer 5 under the Ta layer 9 (the portions shown by a dotted line) is also shaved off.
In the next step shown in FIG. 36, a ferromagnetic layer 11, a second antiferromagnetic layer 12 formed from an Ir—Mn alloy, and an electrode layer 13 are continuously deposited on the free magnetic layer 5 exposed on sideeach side of the resist layer 10. A magnetic sensing element using the exchange bias method is completed by removing the resist layer 10 shown in FIG. 36.
The track width Tw can be defined as a distance between the two ferromagnetic layers 11 (in the X direction). The magnetization of each ferromagnetic layers 11 is tightly fixed in the X direction by the exchange coupling magnetic field generated between the second antiferromagnetic layer 12 and ferromagnetic layer 11 in the magnetic sensing element shown in FIG. 35. Since the magnetization at the side each side A of the free magnetic layer 5 located under the ferromagnetic layer 11 is tightly fixed in the X-direction due to the ferromagnetic coupling between the ferromagnetic layer 11 and free magnetic layer 5, and the central area B of the free magnetic layer 5 in the track width Tw area is considered to be weakly induced into a single magnetic domain state that undergoes fluctuations in the presence of an external magnetic field.
Accordingly, it was expected that the problems described in (1) to (3) might be properly solved by the magnetic sensing element using an exchange bias method.
However, the following additional problems were encountered in the magnetic sensing element manufactured by the manufacturing process shown in FIGS. 35 and 36.
(1) First, not only the Ta layer 9 but also a part of the free magnetic layer 5 is shaved off by ion-milling in the step shown in FIG. 35. In addition, an inert gas such as Ar used for ion-milling tends to diffuse from the surface of the exposed free magnetic layer 5 into the layer. Such damage by ion-milling causes a break in the crystal structure at the surface 5a of the free magnetic layer 5, or generates lattice deficiencies (a mixing effect). Accordingly, magnetic characteristics at the surface 5a of the free magnetic layer 5 tends to deteriorate.
Ideally, only the Ta layer 9 is shaved, or the free magnetic layer 5 is protected from being shaved by leaving the Ta layer behind as a very thin film. However, in practice it is difficult to control ion-milling to an extent as described above.
The difficulty arises from the thickness of the Ta layer 9 formed on the free magnetic layer 5. The Ta layer 9 is deposited with a thickness of about 30 Å to about 50 Å because the free magnetic layer 5 cannot be protected from being oxidized unless it is formed with the above thickness.
The Ta layer 9 is oxidized by exposure to the atmosphere, or by annealing in a magnetic field to generate an exchange coupling magnetic field between the pinned magnetic layer 3 or ferromagnetic layer 11 and antiferromagnetic layer 2 or 12. The thickness of the Ta layer in the oxidized portions is greater than the total thickness of the Ta layer immediately after deposition. For example, suppose that the thickness of the Ta layer 9 is about 30 Å immediately after deposition, then the thickness of the Ta layer 9 increases to about 45 Å due to oxidation.
High energy ion-milling is required to effectively remove the Ta layer 9 made thicker by oxidation. Since high energy ion-milling naturally has a high milling rate, it is almost impossible to stop milling at the moment when the Ta layer 9 having a large thickness has been removed by ion-milling. This means that the margin of milling limit should be wider as the milling energy is higher. Therefore, a part of the free magnetic layer 5 formed under the Ta layer is usually shaved off, and the free magnetic layer 5 tends to suffer excessive damages by high energy ion-milling resulting in substantially degraded magnetic characteristics.
(2) The magnetic characteristics of the surface of the free magnetic layer 5 exposed by ion-milling are degradeddue to the damage caused by ion-milling as described above. Consequently, the ferromagnetic layer 11 should be thicknbecause a magnetic coupling (ferromagnetic exchange coupling interaction) between the free magnetic layer 5 and ferromagnetic layer laminated on the free magnetic layer 5 becomes insufficient.
However, the exchange coupling magnetic field generated between the antiferromagnetic layer 12 and ferromagnetic layer 11 is weakened when the thickness of the ferromagnetic layer 11 is increased. Eventually, magnetization on each outer side of the free magnetic layer 5 cannot be tightly fixed, which gives rise to side-reading. Thus, it is impossible to manufacture a magnetic sensing element that can adequately define the narrow track width.
On the other hand, when the ferromagnetic layer is too thick, an excessive static magnetic field is applied from the inner side of the ferromagnetic layer 11 to the central area B of the free magnetic layer 5, which decreases the sensitivity of the central area B of the free magnetic layer 5 where magnetic inversion is possible.
(3) It is difficult to selectively remove the Ta layer 9 as described above. However, suppose that the Ta layer 9 is left behind on both sides of the free magnetic layer 5, then Ta diffuses into the free magnetic layer and ferromagnetic layer 11 and degrade the magnetic characteristics of the free magnetic layer 5 and ferromagnetic layer 11. As a result, the ferromagnetic coupling between the free magnetic layer 5 and ferromagnetic layer 11 is weakened, and magnetization of the free magnetic layer cannot be properly controlled.
Effective control of magnetization of the free magnetic layer 5 remains impossible in a structure of the magnetic sensing element in which the Ta layer 9 is formed on the free magnetic layer 5, and the ferromagnetic layer 11 and second antiferromagnetic layer 12 are overlaid on the free magnetic layer 5 exposed by shaving both sides of the Ta layer 9. Consequently, a magnetic sensing element capable of properly defining the narrow track width could not be manufactured.